This invention relates generally to a power supply circuit for providing a DC output voltage and is particularly directed to an improved switched-mode power supply for generating plurality of regulated DC output voltages.
In general, a switched-mode power supply includes a switch, an inductor, a rectifier, a capacitor and a load. The load may be considered a resistance in parallel with the capacitor. During that portion of the period in which the switch conducts, a ourrent originating from the input voltage source passes through the inductor resulting in the storage of energy therein. During the other part of the period, in which the switch is rendered nonconducting, the energy stored in the inductor produces a current through the rectifier which recharges the capacitor and replenishes the energy losses due to the load. By controlling the conducting period of the switch relative to this cycle, the output DC voltage across the load can be rendered independent of variations of the input DC voltage which may arise from a variety of factors, including fluctuations in the AC line supply. It is well known in the prior art to employ pulse width modulation (PWM) responsive to an error signal derived from the output voltage to provide a closed regulation loop for maintaining a constant output voltage despite input and/or load variations.
In many systems energized by a DC power supply, it is necessary to provide various regulated DC output voltages to different components in the system. For example, a plurality of precisely defined voltage levels may be required for biasing semiconductor components or for driving logic and/or memory circuitry in a data processing system application. Most prior art switched-mode power supplies offer only one regulated DC output voltage, while the other outputs are semi-regulated by the coupling of the inverter transformer. This provides one well regulated output and other outputs which can vary greatly under various load conditions. If a high level of regulation is required on more than one output, the prior art approach has generally involved either the utilization of exotic and expensive regulator networks for providing multiple regulated outputs, the use of separate regulator networks for each different biasing voltage, or providing a higher input voltage on the inverter transformer in combination with a linear regulator to drop the voltage down to the required level. The first two approaches are complicated and expensive, while the latter approach results in a reduction of power supply efficiency and requires the use of large transistors and heat sinks in higher current applications which also results in a more expensive power supply.
U.S. Pat. No. 4,318,039 to Abbott discloses a DC power supply requiring a complicated and expensive linear regulator circuit requiring as many as 20 semiconductor devices for regulating only one output voltage, with no regulation provided for a second output. U.S. Pat. Nos. 4,253,137 to Rao and 4,327,404 to Horiguchi disclose switched-mode power supply circuits providing only a single DC output voltage. U.S. Pat. No. 4,301,497 to Johari discloses a flyback type DC converter employing a feedback loop to correct for load variations and a forward feed circuit to correct for line voltage variations. The aforementioned patents provide only a single regulated DC output voltage and require complicated and expensive voltage regulation circuitry.
The present invention is intended to overcome the aforementioned limitations of the prior art by providing a switched-mode power supply with output post-regulation capable of operating at reduced output currents and hence increased operating efficiencies while providing a plurality of well regulated DC output voltages.